Atrial and ventricular arrhythmias represent a significant cause of morbidity and mortality in patients with different forms of heart disease. Recent studies have raised widespread concern about the adverse effects of current Class I anti-arrhythmic drugs used to treat these arrhythmias. These concerns have spurred interest in Class III agents whose action is mediated through action potential prolongation, usually via blockade of K+ channels. The affinity of such compounds typically shows a complex dependency upon the conformational state of the channel. Our limited understanding of the action of these agents is limited by our understanding of the voltage dependent and voltage insensitive transitions which accompany activation. A quantitative and molecularly based model of the activation process and its coupling to inactivation is a prerequisite for elucidating the nature of the complex blocking action of Class III agents. Therefore, this proposal seeks to characterize a cardiac transient outward K+ current which plays a significant role in determining human action potential duration. Block of transient outward currents appears to be less likely to induce triggered activity than block of delayed rectifier K+ channels. Because of the unique similarities between the human and ferret cardiac I/to and the near identity of our ferret (FK1) and the human (HK1) clone, our objective will be to develop a molecularly based biophysical model of FK1. This study will combine voltage-clamp, single channel, gating current and mutagenesis studies of FK1 in oocytes and CHO cells for model development. We will test the ability of this model to define the mechanism of action of a closed channel blocking compound (4- aminopyridine). This model will incorporate transitions will correspond to both independent movement of single subunits and cooperative interactions between subunits during the activation process. In activation will be modeled as coupled to activation by electrostatic, hydrophobic and allosteric interactions in an effort to characterize both the development of and recovery from inactivation. Ultimately, elucidation of channel structural and functional features may help identify newer and more efficacious channel blockers.